Add Chinese (zh-CN) UI localization for the GitHub Copilot desktop app

Is your feature request related to a problem? Please describe.
The GitHub Copilot desktop app (standalone agent app, and the terminal/CLI integration) currently only supports an English UI — menus, settings, buttons, and other interface text. There is no option to switch the interface language, even though the underlying model can converse fluently in Chinese.

Describe the solution you would like
Please add an official interface localization option, including Simplified Chinese (zh-CN), for the desktop app UI (menus, settings panel, buttons, tooltips, etc.), separate from the conversational/chat language which already works well in Chinese.

Additional context
This would primarily benefit Chinese-speaking users who use the assistant in Chinese but currently have to navigate an English-only interface. A similar request for Spanish command descriptions was raised in #3681, so there appears to be existing appetite for i18n/localization work in this tool.

中文说明：目前 GitHub Copilot 桌面端的界面（菜单、设置选项、按钮等）仅支持英文，没有切换界面语言的选项。希望官方能够提供包括简体中文在内的界面本地化支持，与已经支持良好的中文对话功能区分开来。

ファイル、テスト、エントリポイントは指定されていません。まずスタンドアロンのデスクトップアプリ UI とターミナル/CLI 統合の領域を特定し、次に、会話言語を分離したまま、インターフェース言語の切り替えでメニュー、設定、ボタン、ツールチップを簡体字中国語に対応させる方法を定義してください。